1. Field of the Invention
The present invention relates to an electrical connection structure between an electrode of a piezoelectric element and a wiring member, and also, to a head suspension employing the electrical connection structure, the head suspension supporting a magnetic head to write and read data to and from a magnetic disk in a magnetic disk drive or hard disk drive.
2. Description of Related Art
Small-sized precision information devices are rapidly advancing, and for use with such devices, needs for micro-actuators capable of conducting positioning control for very small distances are increasing. Such micro-actuators are highly needed by, for example, optical systems for correcting focuses and inclination angles, ink jet printers for controlling ink heads, and magnetic disk drives for controlling magnetic heads.
The magnetic disk drives increase their storage capacity by increasing the number of tracks per inch (TPI) of a magnetic disk, i.e., by narrowing the width of each track on the magnetic disk.
The magnetic disk drive, therefore, needs an actuator capable of precisely positioning the magnetic head within a minute range across tracks.
To meet the need, Japanese Unexamined Patent Application Publication No. 2002-184140 discloses a head suspension with a dual actuator system. The dual actuator system employs a piezoelectric element in addition to a usual voice coil motor that drives a carriage to which the head suspension is attached. The piezoelectric element is arranged between a base plate and a load beam of the head suspension, to minutely move a magnetic head attached to a front end of the load beam.
According to the head suspension of the dual actuator system, the voice coil motor turns the head suspension through the carriage, and in addition, the piezoelectric element deforms in proportion to a voltage applied thereto, to minutely move the magnetic head at the front end of the load beam in a sway direction (a widthwise direction of the load beam) relative to the base plate. With the dual actuator system involving the voice coil motor and piezoelectric element, the head suspension is capable of precisely positioning the magnetic head to a target position on a magnetic disk.
Among the dual-actuator-system head suspensions, some employ an electrical connection structure 101 illustrated in FIG. 11.
In FIG. 11, the electrical connection structure 101 includes an electrode 103a of a piezoelectric element 103 and a terminal 107 of a flexure 105 serving as a wiring member. The electrode 103a and terminal 107 are arranged to face each other and are fixed together.
The flexure 105 has an insulating layer 109 and a wiring pattern 111 that is laid on the insulating layer 109. At the terminal 107 of the flexure 105, the insulating layer 109 has a through hole 113. The through hole 113 defines a connection part 115 where the wiring pattern 111 is exposed toward the piezoelectric element 103. The connection part 115 of the flexure 105 is fixed and connected to the electrode 103a of the piezoelectric element 103 with a conductive adhesive 117.
In the electrical connection structure 101, the connection part 115 of the flexure 105 supplies power through the conductive adhesive 117 to the electrode 103a of the piezoelectric element 103.
When the power is supplied, various parts of the electrical connection structure 101 receive stress due to thermal expansion as illustrated in FIG. 12. In particular, a part between the conductive adhesive 117 and the electrode 103a of the piezoelectric element 103 and a part between the conductive adhesive 117 and the connection part 115 of the terminal 107 of the flexure 105 suffer from relatively high stress due to thermal expansion. In FIG. 12, parts illustrated with darker colors are suffering from higher stress.
The electrode 103a of the piezoelectric element 103 has a rough surface because of a rough surface condition of the piezoelectric element 103, and therefore, a bonding strength between the electrode 103a and the conductive adhesive 117 is relatively high. Accordingly, the part between the electrode 103a of the piezoelectric element 103 and the conductive adhesive 117 is resistive to the relatively high stress caused by thermal expansion.
On the other hand, the surface of the connection part 115 of the terminal 107 of the flexure 105 is smoother than the surface of the electrode 103a of the piezoelectric element 103. A bonding strength between the connection part 115 and the conductive adhesive 117, therefore, is relatively low. Due to this, the conductive adhesive 117 easily peels off the connection part 115 if the stress due to thermal expansion is excessive.